2.2.71
DEVELOPMENT OF AN IMPROVED BRASSICA DIFFERENTIAL SERIES FOR THE IDENTIFICATION OF RACES OF XANTHOMONAS CAMPESTRIS PV. CAMPESTRIS

JG VICENTEl, A IGNATOV2, J CONWAY3, SJ ROBERTS3 and JD TAYLOR3

linstituto Superior de Agronomia, Tapada da Ajuda, 1300 Lisboa, Portugal; 2Institute of Vegetable Breeding and Seed Production, Vniissok, Lesnoy Gorodok, Moscow Region 143080, Russia; 3Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK

Background and objectives
Xanthomonas campestris pv. campestris (X.c.c. ), the causal agent of black rot of crucifers, is one of the most important diseases of Brassica. It occurs world wide and attacks all cultivated brassicas and numerous cruciferous weeds. Control of the disease can be achieved by eliminating sources of inoculum and by the use of resistant cultivars. Breeding for resistance has mainly involved Brassica oleracea. A differential series that separates isolates of X.c.c. into five different races (0-4) has been reported [1]. Differentiation was based mainly on the response of cultivars of B. rapa and B. juncea. Recently, these races were confirmed using an extended differential series that also included cultivars of B. napus and B. oleracea [2]. This study also postulated a gene-for-gene relationship between Brassica cultivars and races of X.c.c.. The objectives of the present study were to extend and improve the differential series for race typing of isolates of X.c.c. and to further elucidate the postulated underlying gene-for-gene relationship controlling the interaction of races and cultivars.

Materials and methods
Tests for pathogenic variation in a collection of isolates of world-wide origin were made in 12 accessions of Brassica spp. including all the previously described differentials [1,2]. A selection of 10 isolates representative of known and potentially different races of X.c.c. was used in screening tests to identify further differentials. Four-week-old plants were inoculated by clipping the edges of leaves with 'mouse tooth' forceps dipped in a suspension of X.c.c.. Approximately 8-12 points of inoculation were made per leaf and three leaves were inoculated per plant. The number of infected points and the severity of symptoms were recorded 2 and 3 ;weeks after inoculation.

Results and conclusions
Isolates representative of four of the known races (0, 1, 2 and 4) were confirmed in the collection of isolates tested. A new accession of B. oleracea, a double-haploid line of cv. Bohmerwaldkohl, was introduced into the differential series allowing the differentiation of two different races previously included in race 1 (designated race 1 A and 1 B). Race-specific resistance was found in two accessions of B. oleracea (Badger Inbred-16 and PI 436606) previously reported as having quantitative resistance. Resistance to race 4 is common in B. rapa and B. napus while resistance to race 1 and 4 is common in B. juncea. An additional B. napus accession (Pi 199947) was also added to the differential series. Some of the accessions previously included in the differential series gave variable reactions, possibly indicating that they were genetic mixtures. Selections within these accessions were made to establish genetically uniform lines. Some accessions may be substituted by double-haploid lines. Crosses between differential lines are being made to obtain information about the inheritance of resistance. A postulated gene-for-gene relationship between X.c.c. races and Brassica cultivars was explained on the basis of 6 matching gene pairs with avirulence (A) genes in pathogen isolates and resistance (R) genes in host cultivars.

References
1. Kamoun S, Kamdar HV, Tola E , Kado C, 1992. Molecular Plant-Microbe Interactions 5, 22-33.
2. Ignatov A, Vicente JG, Conway J, Roberts SJ, Taylor JD, 1997. ISHS Symposium on Brassicas/Tenth Crucifer Genetics Workshop, 215 (Abstr.).